The present invention relates to a variable speed drive.
One known variable speed drive comprises a motor for driving a continuous drive belt that is wound around and movable along an external surface of a circular pyramid. Mounted on the pyramid is the drive""s output shaft. In this arrangement, when the belt is driven around a wide end of the pyramid it causes the pyramid, and so the output shaft, to rotate at a particular speed. By moving the belt towards the apex of the pyramid, the speed at which the pyramid rotates increases. Hence, by suitable positioning the belt relative to the surface of the pyramid, the speed of the output shaft can be varied.
A problem with this drive is that it is not efficient. Another problem is that it is bulky in three dimensions and complex in operation. Whilst various other, more efficient variable speed drives are available, they tend to be complex and expensive.
An object of the present invention is to provide a low cost, low weight and energy efficient variable speed drive.
According to one aspect of the present invention there is provided a variable speed drive comprising a drive, a movable member, an output drive and an endless, extendable, member that extends around the output drive and the movable member, and is driven by the drive, wherein movement of the endless member around the output drive causes said output drive to move and movement of the movable member causes the endless member to expand or contract, thereby varying its overall length, and so varying the speed at which the output drive is driven.
Preferably, two spaced apart movable members are provided, the endless member being located between them, the two members being fixedly mounted relative to each other and movable relative to the drive, such that when the movable members are moved in one direction the endless member is caused to engage a first one of the movable members and is able to move the output drive in a first direction, and when the movable members are moved in another direction the endless member is caused to engage a second one of the members and is able to move the output drive in a second direction, thereby to provide a reversible drive. Preferably, the or each movable member comprises a wheel.
Preferably, the drive comprises a hollow drive shaft. Preferably, the hollow drive shaft is driven by an electric motor. Drive means may be mounted on the hollow drive shaft for interacting with the endless, flexible member, thereby to move it.
Preferably, the endless, extendable member is an endless coil or spring. The endless member may comprise a belt or tube.
When the endless member is a spring or coil, preferably the drive means comprises a lug that depends from an internal wall of the hollow drive shaft for locating between adjacent windings of the spring or coil, so that when the hollow drive shaft is rotating the spring is propelled forward by engagement of successive windings with the rotating lug. Preferably, a pair of lugs is provided, each lug of the pair being between adjacent windings of the spring but offset from the other in such a manner that a first one of the pair contacts a forward one of the adjacent windings and the second one of the pair contacts a rearward one of the windings.
Preferably, two or more lugs are provided opposite each other. This is advantageous because it keeps the spring positioned centrally in the hollow drive shaft. The opposing lugs are preferably positioned so that the angle of incidence of each lug to a winding of the spring or coil can be prevented from becoming too steep. Preferably, each lug is provided with a roller bearing that provides a surface for engagement with the windings of the spring, thereby to reduce the effects of friction.
Preferably, sets of lugs are provided at opposing ends of the hollow drive shaft. This is advantageous because the part of the spring that is between the lugs is substantially unaffected by movement of the or each movable member so that the part that is effected is reduced. This means that the movable member does not need to be moved as far as would otherwise be necessary.
Preferably, a drive wheel is provided on the output shaft, the flexible member being wound around the drive wheel.
Preferably, a detector is provided for detecting expansion or contraction of the endless member, and thereby changes in the speed of the output drive. When the endless member is a spring, preferably the detector is operable to monitor separation of its adjacent windings, thereby to gain a measure of the expansion or contraction of the spring and so its speed. By comparing the separation of the windings of the spring or coil on either side of the output drive, the actual rate of power transmitted can be calculated.
Preferably, a controller is provided for controlling movement of the movable member. Preferably the controller is in communication with the detector so that information relating to a measured speed can be fed back to the controller.
According to another aspect of the present invention there is provided a variable speed drive comprising drive means, an output drive and an endless, extendible member that is driven by the drive means and extends around and drives the output drive, wherein the drive means is operable to drive the endless member at a first speed at a first driving point and means are provided for changing the speed or stopping movement of the endless member at a second point, thereby to cause expansion or contraction of the endless member around the output drive.
The means for changing the speed or stopping movement of the endless member may be operable to reduce the speed of the endless member to substantially zero at the second drive point, so that the endless member is substantially prevented from moving past that point.
Preferably, the endless, extendable member is an endless coil or spring. The endless flexible member may comprise a belt or tube.
When the endless member is a spring or coil, preferably the drive means comprises two hollow drive shafts located on opposing sides of the output drive. Preferably the hollow drive shafts are driven by the same motor.
Preferably, a lug depends from each hollow inner shaft for locating between adjacent windings of the spring or coil, so that when each hollow drive shaft is rotating, the spring is propelled forward by engagement of successive windings with the rotating lug. Preferably, a pair of lugs is provided, each lug of the pair being between adjacent windings of the spring but offset from the other in such a manner that a first one of the pair contacts a forward one of the adjacent windings and the second one of the pair contacts a rearward one of the windings.
Two or more lugs may be provided on opposing sides of the hollow drive. This is advantageous because it keeps the spring positioned centrally in the hollow drive shaft. The opposing lugs are preferably positioned so that the angle of incidence of each lug to a winding of the spring or coil can be prevented from becoming too steep. Preferably, the or each lug is provided with a roller bearing that provides a surface for engagement with the windings of the spring, thereby to reduce the effects of friction.
Preferably, sets of lugs are provided at opposing ends of the hollow drive shaft. This is advantageous because the part of the spring that is between the lugs is substantially unaffected by movement of the or each movable member so that the part that is effected is reduced. This means that the movable member does not need to be pulled as far as would otherwise be necessary.
Preferably, the hollow drive shafts each comprise an inner hollow cylindrical shaft that is releasably coupled to an outer hollow cylindrical shaft, which is driven by a motor. Preferably, the motor is a constant output motor.
Preferably, the means for changing the speed or stopping movement of the extendable member are operable to decouple the inner shaft from its outer shaft, so that the inner shaft no longer rotates and the spring is substantially prevented from moving past that point. Preferably, both of the hollow drive shafts are driven by the same motor.
The means for changing the speed or stopping movement of the extendable member may comprise a braking mechanism for braking movement of at least one of the inner shafts, thereby decoupling it from the corresponding outer shaft, so that the speed of the endless spring through that shaft can be reduced or even brought to zero.
The means for changing the speed or stopping movement of the extendable member may comprise a compression spring that is mounted between one end of the inner drive shaft and the same end of the outer drive shaft, wherein the compression spring is operable to pull the inner shaft into driving engagement with the outer shaft until a pre-determined tension of the endless member is reached, at which stage the pulling action of the member overcomes the pulling action of the compression spring, so that the inner shaft is moved out of engagement with the outer shaft.
Preferably, there is provided an actuator that is operable to sense when it is time to switch off the drive and move one of the inner shafts against the action of the compression spring and out of engagement with the outer shaft, thereby to decouple that inner shaft from the outer shaft in order to loosen off the spring. A detector may be provided to detect a loosening of the spring and switch off the motor that drives the outer shafts, thereby to return the drive to a starting position.
The inner shaft may have two separate but interlocking parts that can move longitudinally apart from each other, each of the parts being releasably mounted in driving engagement with the outer drive shaft, wherein the means for changing the speed or stopping movement of the endless member past the second point comprises means for moving the parts of the inner shafts from their driven position to a position in which they are de-coupled from the outer shaft. Preferably, the means for moving the parts of the inner shaft comprise elliptical wheels that are rotatable into engagement with the parts of the inner shaft and thereby push the parts away from each other and out of driving engagement with the outer shaft.
Preferably, a drive member is provided on the output drive, the flexible member being wound around the drive member, which drive member may be circular or, for example xe2x80x9cpearxe2x80x9d shaped or elliptical.
Preferably, a detector is provided for detecting the expansion or contraction of the flexible member, and thereby changes in the speed of the output drive. When the flexible member is a spring, preferably the detector is operable to monitor separation of its adjacent windings, thereby to gain a measure of the expansion or contraction of the spring and so its speed. By comparing the separation of the windings of the spring or coil on either side of the output drive, the actual rate of power transmittal can be calculated.
Preferably, a controller is provided for controlling movement of the movable member. Preferably the controller is in communication with the detector so that information relating to a measured speed can be fed back to the controller.